1. Field of the Invention
This invention relates generally to a system wherein information is transferred between a communications controller and a number of work stations, each of which may include a keyboard and a cathode ray tube display; and more particularly to the logic in the communications controller which receives the information data bit stream from a work station and synchronizes the data bit stream with the communications controller clocking system.
2. Description of the Prior Art
Information being transferred from a remote work station to a controller over a single conductor is received as a serial string of data bits. The serial string may be made up of asynchronous characters, that is, having a start bit, a number of data bits, and one or two stop bits. A clocking system operating at 16 times the bit rate locates the center of the start bit and strobes the center of the subsequent data and stop bits. Such a system is described in U.S. Pat. No. 3,680,057 entitled "Data Communications Subchannel".
The start and stop bits may be removed and the data bits sent as a synchronous stream to increase the character rate. In communications systems which include modems, the modem generates a separate clock which is received by the controller and samples the data bits. This is described in the aforementioned U.S. Patent.
It was desirable, however, to eliminate the separate clock signal and replace it with clocking logic internal to the controller. The clocking logic generated a clocking signal 16 times the data bit rate and a particular clock signal was selected to sample the midpoint of each of the synchronous data bits.
U.S. application Ser. No. 194,495 filed Oct. 6, 1980, entitled "A Data Processing System Having Apparatus in a Communications Subsystem for Establishing Byte Synchronization" describes apparatus for synchronizing the byte timing signal to the byte. A microprocessor generates a fixed format of binary ONE's and binary ZERO's to determine the number of bits between the byte timing signal and the start of the byte.
U.S. application Ser. No. 304,754 filed Sept. 23, 1981, entitled "Digital Serial Interface and Receive Logic" describes apparatus for synchronizing the serial data bits to the communications controller. The instant invention is an improvement of the synchronization apparatus.
However, as the demand for communicating with a larger number of work stations grew, the need for a higher data bit rate and an improved means of synchronizing the data to the controller clock was needed to assure that the data was received with a reduced number of errors and thereby reducing the number of retransmission requests.